Some crystalline aluminosilicates, or zeolites, are useful as adsorbents in separating a particular hydrocarbon compound from hydrocarbon mixtures containing that compound. In particular, zeolites are widely used for selective separation of paraxylenes from mixtures containing other C.sub.8 aromatic compounds such as metaxylene, orthoxylene, or ethylbenzene. For example, U.S. Pat. Nos. 3,636,121; 3,686,342; 3,686,343; 3,835,043; 3,855,333; 3,878,127; 3,894,108; 3,903,187 and 4,265,788 are all directed towards methods of removing paraxylene from mixtures or of selectively separating paraxylene and ethylbenzene from mixtures containing other components, using various types of zeolites as adsorbents. Paraxylene is a commercially important aromatic hydrocarbon isomer since its use in the manufacture of terephthalic acid is a critical step in the subsequent production of various fibers such as Dacron.
This invention however relates to a process for separating ethylbenzene from a feed mixture containing ethylbenzene and at least one other xylene isomer and is therefore unrelated to paraxylene isomer separation processes. Additionally, in the process disclosed herein, ethylbenzene is selectively adsorbed in relation to the less selectively adsorbed xylene isomers.
While a separation of paraxylene from other xylene isomers is desirable in certain circumstances, it has become increasingly desirable to recover ethylbenzene from streams containing both ethylbenzene and xylene isomers. Ethylbenzene has great commercial importance since it is a building block in the production of styrene. Further, the cost of producing ethylbenzene by the reaction of benzene with ethylene has steadily increased. These costs have prompted research efforts in the recovery of ethylbenzene from various C.sub.8 aromatic feedstreams which already contain ethylbenzene. Such feedstreams may be C.sub.8 aromatic extracts resulting from various solvent extraction processes, or pyrolysis gasoline or reformed naphtha.
It is known that potassium substituted type Y zeolites selectively adsorb ethylbenzene from mixtures comprising ethylbenzene, metaxylene and orthoxylene using toluene as a desorbent. See for instance, U.S. Pat. No. 4,031,156. U.S. Pat. No. 3,998,901 suggests that ethylbenzene can be separated from xylene isomers using a type Y zeolite substituted with Sr and K wherein ethane or toluene is used as the desorbent. In U.S. Pat. No. 3,943,182 desorbents other than toluene are used as desorbents in selectively separating ethylbenzene from xylenes by use of a type X zeolite.
However, U.S. Pat. No. 3,686,342 suggests that type X and type Y zeolites are substantially equivalent in showing a preferred selectivity for paraxylene with respect to ethylbenzene, particularly when substituted with metals such as, inter alia, rubidium and cesium. Nonequilibrium absorption conditions are said to be the reason for the selective adsorptivity. In contrast, other U.S. patents suggest that RbX and CsX zeolites are known to adsorb selectively ethylbenzene over paraxylene. U.S. Pat. Nos. 3,943,182; 4,031,156; and 4,175,099 show RbX zeolites in such service and U.S. Pat. Nos. 3,867,470 and 3,943,182 show CsX zeolites. Consequently, the selectivity of such zeolites is not readily predictable and may in some instances depend upon the choice of desorbents for their selectivity. Moreover, the direction in which a zeolite's specific adsorptivity moves as a result of using a particular desorbent is even less predictable.